Can I disable dual monitors and go back to a clone?

I hae an HP xp 4300 workstation with 2 monitors. The dual monitor features works just fine. Sometimes, I'd like both monitors to display the same thing for training purposes. Is there any way to turn off the "Extend my screen?" When I uncheck the box in the display settings, the 2nd monitor just goes black and says "no signal".

Yes. Use Ultra-Mon http://www.realtimesoft.com/ultramon/ It can turn on and off your secondary monitor so that it will act as if the computer was not on.It a a lot of other awesome features which I myself use and love. I don't know of any other way though sorry.